A short story that was origionaly used to promote The Titan's Curse and it was only availble through a book store mailing list, but Rick put it up on his blog in three parts so everyone could read it!!
Here's the link: http://rickriordan.blogspot.com/search?q=the+stolen+chariot
The story takes place between book three and four. It is courrently a Boarders exclusive, but here's the link: http://www.borders.com/online/store/ArticleView_percyjacksonandbronzedragon